Waveguide sheet and methods for manufacturing the same

ABSTRACT

In one aspect, an illumination structure includes a substantially non-fiber waveguide, which itself includes a discrete in-coupling region for receiving light, a discrete propagation region for propagating light, and a discrete out-coupling region for emitting light.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to and the benefit of U.S. ProvisionalPatent Application No. 61/006,110, filed on Dec. 19, 2007; U.S.Provisional Patent Application No. 61/064,384, filed on Mar. 3, 2008;U.S. Provisional Patent Application No. 61/127,095, filed on May 9,2008; U.S. Provisional Patent Application No. 61/076,427, filed on Jun.27, 2008; and U.S. Provisional Patent Application No. 61/135,098/, filedon Jul. 16, 2008. The entire disclosure of each of these applications isincorporated by reference herein.

TECHNICAL FIELD

In various embodiments, the present invention relates to optics, and inparticular to optical waveguides.

BACKGROUND

The technology to transmit and guide light through optical systemsexploits a physical phenomenon in which light is confined within amaterial surrounded by other materials with lower refractive index. Suchoptical systems are generally referred to as optical waveguides, and areemployed to direct, diffuse, and/or polarize light in many applications,e.g., optical communication and illumination.

When a ray of light moves within a transparent substrate and strikes oneof its internal surfaces at a certain angle, the ray of light is eitherreflected from the surface or refracted into the open air in contactwith the substrate. The condition according to which the light isreflected or refracted is determined by Snell's law, which relates theimpinging angle, the refracting angle (in the case of refraction) andthe refractive indices of both the substrate and the air. Broadlyspeaking, depending on the wavelength of the light, for a sufficientlylarge impinging angle (above the “critical angle”) no refraction occurs,and the energy of the light is trapped within the substrate. In otherwords, the light is reflected from the internal surface as if from amirror. Under these conditions, total internal reflection is said totake place.

Many optical systems operate according to the principle of totalinternal reflection. Optical fiber represents one such system. Opticalfibers are transparent, flexible rods of glass or plastic, basicallycomposed of a core and cladding. The core is the inner part of thefiber, through which light is guided, while the cladding surrounds itcompletely. The refractive index of the core is higher than that of thecladding, so that light in the core impinging the boundary with thecladding at an angle equal to or exceeding the critical angle isconfined in the core by total internal reflection. Thus, geometricoptics may be used to derive the largest angle at which total internalreflection occurs. An important parameter of every optical fiber (or anyother light-transmitting optical system) is known as the “numericalaperture,” which is defined as the sine of the largest incident lightray angle that is successfully transmitted through the optical fiber,multiplied by the index of refraction of the medium from which the lightray enters the optical fiber.

Another optical system designed for guiding light is the graded-indexoptical fiber, in which the light ray is guided by refraction ratherthan by total internal reflection. In this optical fiber, the refractiveindex decreases gradually from the center outwards along the radialdirection, and finally drops to the same value as the cladding at theedge of the core. As the refractive index does not change abruptly atthe boundary between the core and the cladding, there is no totalinternal reflection. However, the refraction nonetheless bends theguided light rays back into the center of the core while the lightpasses through layers with lower refractive indices.

Another type of optical system is based on photonic materials, wherelight is confined within a bandgap material surrounding the light. Inthis type of optical system, also known as a photonic materialwaveguide, the light is confined in the vicinity of a low-index region.One example of a photonic material waveguide is a silica fiber having anarray of small air holes throughout its length.

International Patent Application Publication No. WO2004/053531, theentire contents of which are hereby incorporated by reference, disclosesa waveguide for propagating and emitting light. The waveguide is made ofa flexible, multilayer waveguide material in which the refractive indexof one layer is larger than the refractive index of the other layers toallow propagation of light via total internal reflection. One layer ofthe waveguide material comprises one or more impurities which scatterthe light to thereby emit a portion thereof through the surface of thewaveguide material.

Impurities for light scattering are also employed in light diffusers(also known as light-scattering films or diffusing films), which diffuselight from a source in order to attain a uniform luminance. For example,in a liquid crystal display device a light diffuser is placed betweenthe light source or light reflector and the liquid crystal panel so asto diffuse the illuminating light, allowing the device to be used as aplane or flat light source as well as enhancing the luminance on thefront side of the device.

Conventional illumination apparatuses capable of emitting diffused lightwith uniform luminance are complicated to manufacture and too large formany applications. They tend to be unitary and large rather than smalland scalable. Additionally, such apparatuses often exhibit insufficientcolor mixing and diffusion to emit light with a high degree of color andluminance uniformity.

SUMMARY

The foregoing limitations of conventional illumination apparatuses areherein addressed by utilizing a waveguide that incorporates in-coupling,propagation, and out-coupling regions and/or that is easily manufacturedas a group of aligned core structures.

Generally, embodiments of the invention propagate and diffuse lightuntil it exits though a surface of the waveguide device or a portionthereof. The light path may involve two right angles: in variousembodiments, light is absorbed into the structure through the bottomsurface of one portion the waveguide (e.g., the in-coupling region) andis emitted from a top surface of a second portion of the waveguide(e.g., the out-coupling region). These waveguide portions havesubstantially no overlap; they may be separated, for example, by apropagation region from which light is not emitted.

In various embodiments, light entering a waveguide's in-coupling regionis substantially retained within the waveguide until it is emitted fromthe out-coupling region. The different emission and retention behaviorof the various waveguide portions may be obtained using differentconcentrations of scattering particles; for example, the propagationregion may be devoid of scattering particles altogether in order to keeplight confined therein.

Embodiments of the invention successfully provide an optical waveguidedevice that may be tiled or overlapped. As further detailed herein, theoptical properties of the waveguide may be tailored to the requirementsof particular applications.

The design of waveguide-based light structures in accordance with theinvention also facilitates convenient manufacture. The light structuremay, for example, be assembled by joining a plurality of corestructures, each of which has a different concentration of scatteringparticles (or no scattering particles at all). Forming the joined corestructures may be accomplished by, e.g., co-injection molding,coextrusion, coating, lamination, bonding, and/or welding.

In an aspect, embodiments of the invention feature an illuminationstructure including a substantially non-fiber waveguide and a discretelight source disposed proximate a bottom surface of a first portion ofthe waveguide. Light is absorbed into the illumination structure throughthe bottom surface of the first portion and is emitted from a topsurface of a second portion of the waveguide; the second portion hassubstantially no overlap with the first portion of the waveguide. Thefirst and second portions of the waveguide may be spaced apart from eachother. In general, light is emitted only from the second portion of thewaveguide.

In another aspect, embodiments of the invention feature a substantiallynon-fiber waveguide and a discrete light source disposed proximate abottom surface of a first portion of the waveguide. A propagationdirection, within a second portion of the waveguide, of light from thediscrete light source is substantially perpendicular to an in-couplingdirection of the light. The propagation direction of the light may besubstantially perpendicular to an out-coupling direction of the light ina third portion of the waveguide. The illumination structure may includea phosphor material for converting some of the light to a differentwavelength, the converted light mixing with unconverted light to formmixed light spectrally different from both the unconverted light and theconverted light. An out-coupling direction of the mixed light may besubstantially perpendicular to the propagation direction of light fromthe discrete light source.

In yet another aspect, embodiments of the invention feature a method offorming a substantially non-fiber waveguide. The method includes forminga plurality of joined core structures, at least one of which issubstantially free of scattering particles. At least some of the corestructures may include pluralities of scattering particles, and thesize, the concentration, and/or the type of the scattering particles mayvary among at least two of the core structures. Forming the plurality ofjoined core structures may include or consist essentially of at leastone of co-injection molding, coextrusion, coating, lamination, bonding,or welding. The method may include forming a cladding layer on the topsurface and/or the bottom surface of the plurality of joined corestructures. There may be substantially no overlap between adjoining corestructures across a thickness thereof. The waveguide may besubstantially planar.

These and other objects, along with advantages and features of thepresent invention herein disclosed, will become more apparent throughreference to the following description, the accompanying drawings, andthe claims. Furthermore, it is to be understood that the features of thevarious embodiments described herein are not mutually exclusive and mayexist in various combinations and permutations.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings, like reference characters generally refer to the sameparts throughout the different views. Also, the drawings are notnecessarily to scale, emphasis instead generally being placed uponillustrating the principles of the invention. In the followingdescription, various embodiments of the present invention are describedwith reference to the following drawings, in which:

FIG. 1 a is a schematic illustration showing a perspective view of anoptical waveguide device which comprises a plurality of core structuresjoined in a side-by-side configuration, according to some embodiments ofthe present invention;

FIG. 1 b is a schematic illustration showing a cross-sectional viewalong the line A-A of FIG. 1 a, according to some embodiments of thepresent invention;

FIG. 1 c is a schematic illustration showing a perspective view of thedevice optical waveguide device in embodiments in which the devicecomprises one or more cladding layers;

FIGS. 1 d and 1 e are schematic illustrations showing cross sectionalviews along line B-B of FIG. 1 c;

FIGS. 1 f and 1 g are schematic illustrations showing perspective viewsof the optical waveguide device without (FIG. 1 f) and with (FIG. 1 g)claddings, in an embodiment in which the core layer of the device isformed of core structures in the shape of plaques;

FIG. 1 h is a schematic illustration showing a perspective view of anoptical waveguide device which comprises a plurality of core structuresjoined in a nested configuration, according to some embodiments of thepresent invention;

FIGS. 2 a and 2 b show representative examples of an optical mean freepath as a function of a lateral direction, according to some embodimentsof the present invention;

FIGS. 3 a-3 d are schematic illustrations showing fragmentarycross-sectional views of the device, according to some embodiments ofthe present invention;

FIGS. 4 a-4 d are schematic illustrations showing cross-sectional viewsof an optical funnel, according to some embodiments of the presentinvention;

FIGS. 4 e-4 g are cross-sectional views of an optical waveguide devicehaving substantially no line-of-sight between a photoluminescentmaterial and a light-emitting element in an optical funnel, according tosome embodiments of the present invention;

FIGS. 4 h-4 j are cross-sectional views of an optical waveguide devicehaving substantially no line-of-sight between a photoluminescentmaterial and a light-emitting element embedded within the waveguidedevice, according to some embodiments of the present invention;

FIG. 5 is a schematic illustration of a coextrusion apparatus forforming a core layer, according to some embodiments of the presentinvention;

FIG. 6 is a schematic illustration of a coextrusion apparatus forforming a core layer and one or more cladding layers, according to someembodiments of the present invention;

FIGS. 7 a and 7 b are schematic illustrations of a process for forming acore layer and optionally one or more cladding layers using extrusioncoating technique, according to various exemplary embodiments of thepresent invention;

FIGS. 8 a-8 c are schematic illustrations of a process for forming acore layer and optionally one or more cladding layers using laminationtechnique, according to various exemplary embodiments of the presentinvention;

FIGS. 9 a-9 c are schematic illustrations of a process for forming acore layer and optionally one or more cladding layers using tilingtechnique, according to various exemplary embodiments of the presentinvention;

FIGS. 10 a-10 c are schematic illustrations of a process formanufacturing a core layer by co-injection technique, according tovarious exemplary embodiments of the present invention;

FIG. 11 is a schematic illustration showing a perspective view of amultilayer optical waveguide device, according to some embodiments ofthe present invention;

FIG. 12 a is a plan view of an illumination panel incorporating multipleoptical waveguide devices, according to some embodiments of the presentinvention;

FIG. 12 b is an exploded view of a display device incorporating theillumination panel depicted in FIG. 12 a; and

FIG. 13 is a schematic illustration of a light-emitting element thatincludes a phosphor layer.

DETAILED DESCRIPTION

FIGS. 1 a-1 h illustrate an optical waveguide device 10 according tovarious exemplary embodiments of the present invention. Device 10generally has an open shape (i.e., non-tubular) such as the shape of asheet, e.g., a planar sheet. Typically, device 10 is a non-fiber device,i.e., not a substantially cylindrical waveguide in which alight-conducting core is surrounded by a layer of cladding, and is solid(i.e., not hollow). In various exemplary embodiments of the inventionthe sheet is flexible and hence may be also assume a non-planar shape.For example, when the sheet is placed on a curved surface, the sheet mayacquire the curvature of the surface. Device 10 may also have a certaindegree of elasticity. Thus, one or more of the layers of device 10 maybe made, for example, from an elastomer. In some embodiments, device 10is substantially rigid.

Device 10 includes a core layer 16 formed of a plurality of corestructures 18 joined in, e.g., a side-by-side or nested configuration.Core structures 18 (designated in FIGS. 1 a-1 h by reference numerals18-1, 18-2, . . . , 18-n) may take the form of elongated bands arrangedside-by-side (see, e.g., FIG. 1 a), or may instead have a nestedconfiguration (see, e.g., FIG. 1 h) with their common ends joined. Thewidth of structures 18 (along the x direction in FIGS. 1 a-1 h) mayvary.

For clarity of presentation device 10 is shown as planar, with layer 16being parallel to the x-y plane, and each elongated core structure 18extending along the y direction. The ordinarily skilled person wouldknow how to tailor the following description for non-planar devices. Forexample, each section of a non-planar sheet may be described using aCartesian x-y-z coordinate system which is rotated such that the sectionis tangential to the x-y plane and normal to the z direction. The x, yand z directions are referred to herein as the “lateral,” “longitudinal”and “normal” directions, respectively.

Although core structures 18 are shown in FIGS. 1 a-1 g as having arectangular cross-section, this need not necessarily be the case,depending on the application.

With more specific reference to FIGS. 1 a and 1 b, FIG. 1 a is aperspective view of device 10 and FIG. 1 b is a cross-sectional viewalong the line A-A of FIG. 1 a in embodiments in which core layer 16 isat least partially surrounded by air. FIG. 1 b is a cross-sectional viewalong the line A-A of a device similar to that illustrated FIGS. 1 a and1 h in embodiments in which core layer 16 is at least partiallysurrounded by air.

In embodiments in which core layer 16 is at least partially surroundedby air, structures 18 are characterized by a refractive index which islarger than the refractive index of the surrounding air. In suchconfiguration, when light strikes the internal surface of layer 16 at anangle larger than the critical angle, θ_(c≡sin) ⁻¹(n₁/n₂), where n₁ andn₂ are the refractive indices of the air and the core layer,respectively, the light energy is trapped within core layer 16 andpropagates therethrough via total internal reflection. Light may alsopropagate through device 10 when the impinging angle is smaller than thecritical angle, in which case one portion of the light is emitted andthe other portion thereof continues to propagate. The difference betweenthe indices of refraction the core layer and surrounding air may beselected in accordance with the desired propagation angle of the light.

Typically, the refractive index of air is about 1; hence, corestructures 18 typically include or consist essentially of a waveguidematerial having a refractive index greater than 1. Representativeexamples of waveguide materials suitable for the core structuresinclude, without limitation, a thermoplastic such as a polycarbonate,polymethyl methacrylate (PMMA), and/or polyurethane (TPU) (aliphatic)with a refractive index of about 1.50, TPU (aromatic) with a refractiveindex of from about 1.58 to about 1.60, amorphous nylon such as GRILAMIDsupplied by EMS Grivory (e.g., GRILAMID TR90 with refractive index ofabout 1.54), polymethylpentene, e.g., TPX supplied by Mitsui with arefractive index of about 1.46, polyvinylidene fluoride (PVDF) with arefractive index of about 1.34, or other thermoplastic fluorocarbonpolymers, and/or STYROLUX (UV stabilized) supplied by BASF withrefractive index of about 1.58.

FIGS. 1 c-1 e are perspective (FIG. 1 c) and cross-sectional (FIGS. 1 dand 1 e) views along line B-B of device 10 in embodiments in whichdevice 10 further comprises one or more cladding layers. Although thecore layer of the device shown in FIG. 1 c is illustrated as having thecore structures in a side-by-side configuration, this need notnecessarily be the case, since, for some applications, it may be desiredto have the core layer arranged in a nested configuration (e.g., theconfiguration schematically illustrated in FIG. 1 h).

As shown in FIGS. 1 c-1 e, device 10 includes a first cladding layer 12,a second cladding layer 14, and core layer 16 interposed betweencladding layers 12, 14. Typically, the elongated structures of core 16extend along the length of the cladding layers.

The refractive index of the cladding layers is typically smaller thanthe refractive index of the core layer. As a result, when light strikesthe internal surface of the cladding layers at an impinging angle largerthan the critical angle (θ_(c)≡sin⁻¹(n₁/n₂), where n₁ and n₂ are therefractive indices of the cladding and core layers, respectively), thelight energy is trapped within core layer 16, and the light propagatestherethrough. The light may also propagate through device 10 when theimpinging angle is smaller than the critical angle, in which case oneportion of the light is emitted and the other portion continues topropagate. The difference between the indices of refraction of thelayers is preferably selected in accordance with the desired propagationangle of the light.

In the embodiments in which the cladding layers are employed, corestructures 18 include or consist essentially of a waveguide materialsuch as those identified above for the embodiment lacking claddinglayers, and preferably have relatively high refractive indices.

In accordance with embodiments of the present invention, the indices ofrefraction are selected such that propagation angle is from about 2° toabout 55°. For example, core layer 16 may be made of GRILAMID TR90 witha refractive index of about 1.54, and cladding layers 12, 14 may be madeof TPX with refractive index of about 1.46, so that Δn≡n₂−n₁≈0.08 andn₁/n₂≈0.948, corresponding to a propagation angle of 90°−sin⁻¹(0.948),or approximately ±19°. In another example, a core layer 16 made of TPU(aromatic) with a refractive index of about 1.60 without cladding has acorresponding propagation angle of 90°−sin⁻¹(1/1.6), or approximately±51°.

In some embodiments of the invention, core structures 18 do not haveelongated shapes. FIGS. 1 f and 1 g are perspective views of device 10without (FIG. 1 f) and with (FIG. 1 g) claddings, in an embodiment inwhich core structures 18 are in the shape of plaques (e.g., polygonalplaques such as squares or rectangles). The ordinarily skilled personwill know how to construct a cross-sectional view of theseillustrations, which may be similar to FIGS. 1 b, 1 d and 1 e.

The partitioning of core layer 16 into core structures 18 (elongated orshaped as plaques) may be accomplished by any process known in the art,such as, but not limited to, coextrusion, extrusion, coating,coinjection molding, lamination, tiling, and the like. For example, twoadjacent structures may be welded at their joined ends, bonded by anadhesive material disposed along their length and/or width, etc. Aprocess for forming core layer 16 according to some embodiments of thepresent invention is provided below.

Whether or not device 10 includes cladding layers, and irrespectively ofthe shape and arrangement of the core structures forming layer 16, someof the core structures include additives selected to provide theindividual core structures with a predetermined effective refractiveindex. The effective refractive index depends on the type andconcentration of the additive. Typically, higher additive concentrationsprovide higher effective refractive indices. The additives may take theform of light-scattering particles 20 embedded in one or more of thecore structures. In various exemplary embodiments of the invention, thesize, concentration, refractive index, and/or type of light-scatteringparticles 20 varies among at least two of the core structures.

Particles 20 are dispersed within core structures 18 and facilitateemission of the light from a surface 23 of core layer 16 and/or asurface 24 of cladding layer 14 (in the embodiments in which claddinglayer 14 is employed). Particles 20 serve as scatterers and typicallyscatter optical radiation in more than one direction. When light isscattered by a particle 20 such that the impinging angle is below thecritical angle, no total internal reflection occurs and the scatteredlight is emitted through surface 23 and/or surface 24.

The light-scattering particles may be beads, e.g., glass beads, or otherceramic particles, rubber particles, silica particles, particlesincluding or consisting essentially of inorganic materials such as BaSO₄or TiO₂, particles including or consisting essentially of a phosphormaterial (as further described below), and the like. In an embodiment,the light-scattering particles are substantially or even completelynon-phosphorescent. Such non-phosphorescent particles merely scatterlight without converting the wavelength of any of the light striking theparticles. The term “light-scattering particles” may also refer tonon-solid objects embedded in the waveguide material from which corestructure are made, provided that such objects are capable of scatteringthe light. Representative example of suitable non-solid objects include,without limitation, closed voids within the core structures, e.g., airbubbles, and/or droplets of liquid embedded within the core structures.The light-scattering particles may also be organic or biologicalparticles, such as, but not limited to, liposomes. In some embodiments,optical elements such as microlenses are utilized in conjunction with,or even instead of, light-scattering particles. In other embodiments,optical elements include or consist essentially of structures such ashemispheres or diffusive dots. In such embodiments, the optical elementsfunction to out- couple light propagating through device 10. As utilizedherein, “optical elements” may generically refer to elements such asmicrolenses as well as light-scattering particles, e.g.,non-photoluminescent particles.

In accordance with various embodiments of the present invention, theconcentration, size and/or type of particles is selected such as toprovide illumination at a predetermined profile (e.g., intensityprofile) from predetermined regions of surface 23 or 24. For example, inregions of device 10 where a larger portion of the propagated light isto be emitted through the surface, the concentration of particles 20 maybe large and/or the particles may have a type and/or size which providesthem with high scattering properties; in regions where a smaller portionof the light is to be emitted the concentration of particles 20 may besmaller and/or the particles may have a type and/or size which providesthem with lower scattering properties; and in surface regions from whichno light is to be emitted, substantially no particles are embedded incore structures 18.

As will be appreciated by one ordinarily skilled in the art, the energytrapped in waveguide device 10 decreases each time a light ray isemitted through surface 23 or 24. On the other hand, it may be desiredto use device 10 to provide a uniform surface illumination. Thus, as theoverall amount of energy decreases with each emission, a uniform surfaceillumination may be achieved by gradually increasing the ratio betweenthe emitted light and the propagated light. According to someembodiments of the present invention, the increasing ratio of emittedlight to propagated light is achieved by an appropriate selection of thedistribution, type, refractive index, and/or size of particles 20 in thecore layer 16. For example, at regions in which it is desired to haveuniform surface illumination, the concentration of particles 20 may bean increasing function of the optical distance traversed by thepropagated light.

Generally, the optical output at specific and predetermined regions maybe controlled by arranging the core structures 18 such that differentcore structures have different concentrations, sizes, refractiveindices, and/or types of particles 20.

In various exemplary embodiments of the invention, the core structures18 are arranged to define a first zone 26 and a second zone 28. Firstand second zones 26, 28 may include portions of core layer 16 such thata profile of an optical mean free path characterizing core layer 16 isgenerally flat across the first zone 26 and monotonically varying acrossthe second zone 28.

The optical mean free path may be measured directly by positioning abulk material in front of a light-emitting element and measuring theoptical output through the bulk at a given direction as a function ofthe thickness of the bulk. Typically, when a bulk material, t mm inthickness, reduces the optical output of a light source at the forwarddirection by 50%, the material is said to have a mean free path of t mm.

FIG. 2 a shows a representative example of an optical mean free path asa function of the lateral direction x. As shown, the optical mean freepath is substantially constant in zone 26, and is a decreasing functionof x in zone 28. The decrement of the optical mean free path in region28 facilitates an increasing ratio between the emitted portion andpropagated portions of the light.

Zone 26 may include one or more core structures and is typically devoidof light-scattering particles 20. In this embodiment, zone 26 propagateslight with minimal or no emissions from surfaces 23 or 24, i.e., zone 26is a propagation region. Zone 28 may include a plurality, e.g., three ormore, of core structures 18 each having particles 20 embedded therein.In such an embodiment, zone 28 provides illumination by out-couplinglight from core 16 (i.e., zone 28 is an out-coupling region for lightpropagated through zone 26). The brightness of the illumination fromzone 28 may be substantially uniform.

Brightness uniformity may be calculated by considering the luminancedeviation across the range of azimuthal angles as a fraction of theaverage luminance across that range. A more simple definition of thebrightness uniformity BU is BU==1−(L_(MAX)−L_(MIN))/(L_(MAX)+L_(MIN)),where L_(MAX) and L_(MIN) are, respectively, the maximal and minimalluminance values across the predetermined range of azimuthal angles.

The term “substantially uniform brightness” refers to a BU value whichis at least 0.8 when calculated according to the above formula. In someembodiments of the invention the value of BU is at least 0.85, morepreferably at least 0.9, and still more preferably at least 0.95.

To achieve a decreasing optical mean free path, the concentration ofparticles 20 in the core structures 18 of zone 28 may be an increasingfunction of the distance from zone 26. Alternatively or additionally,the type and/or size of the particles in the individual core structures18 of zone 28 may vary to achieve the desired profile. As shown in FIG.1 b, the concentration, type, size, and/or refractive index of particles20 in zone 28 may change in a direction of light propagation throughdevice 10 (denoted as the x direction in FIG. 1 b). However, for anycross-section through zone 28, the concentration, type, size, and/orrefractive index of particles 20 may be substantially constant in atleast one of the directions perpendicular to the light-propagationdirection (e.g., they and z directions in FIG. 1 b). For example, eachcore structure 18 in zone 28 may have a substantially constantconcentration, type, size, and/or refractive index of particles 20therewithin, but this value may change in at least one (or every) othercore structure 18 in zone 28.

Some embodiments of the present invention include a third zone 30. Asshown in FIG. 1 a, third zone 30 may be proximate or in direct contactwith first zone 26 and away from second zone 28. Third zone 30 maycomprise or consist essentially of one or more core structures 18 havinglight-scattering particles 20 embedded therein. A representative exampleof an optical mean free path in the embodiment in which three zones aredefined is illustrated in FIG. 2 b.

Zone 30 may be an in-coupling region for facilitating the entry of lightinto device 10. Light enters device 10 at zone 30, propagates throughzone 26 and exits (i.e., is out-coupled) at zone 28. One or more of thecore structures 18, typically the first and last structures (i.e.,structures 18-1 and 18-n in the illustration of FIG. 1 d) may be madelight-reflective so as to prevent or reduce optical losses through theside(s) of device 10. The characteristic refractive index of suchlight-reflective core structures 18 is preferably above 2. Arepresentative example of a material having a sufficiently highrefractive index suitable for the present embodiment is TiO₂, which hasa refractive index of about 2.5. Alternatively, light-reflectivestructures 22 may be disposed proximate the entire height of device 10as shown in FIG. 1 e.

Coupling of light into device 10 may be facilitated using an opticalfunnel 32 positioned adjacent to layer 12 or layer 16 at zone 30. Funnel32 is preferably configured to receive light from one or morelight-emitting elements and to transmit the light into layer 12 or layer16. The principle of operation of funnel 30 according to someembodiments of the present invention is further detailed herein underwith reference to FIG. 4.

To prevent or reduce optical losses through the portion of claddinglayer 14 which overlaps zone 30, device 10 may further include one ormore light reflectors 36 adjacent to cladding layer 14 at the region ofcladding layer 14 which overlaps zone 30. Reflector(s) 36 reduceillumination in any direction other than a circumferential direction.

In various exemplary embodiments of the invention, zone 30 of device 10includes one or more components that cause the light exiting zone 30(into zone 26) to have a predetermined optical profile, such as, but notlimited to, a substantially uniform color profile or substantiallyuniform white light. This embodiment may be implemented by color mixing,optical means, or may be implemented via luminescence, a phenomenon inwhich energy is absorbed by a substance, commonly called a luminescent,and is emitted in the form of light. The wavelength of the emitted lightdiffers from the characteristic wavelength of the absorbed energy (thecharacteristic wavelength equals hc/E, where h is the Plank's constant,c is the speed of light and E is the energy absorbed by theluminescent). Luminescence is a widely occurring phenomenon which may beclassified according to the excitation mechanism as well as according tothe emission mechanism. Examples of such classifications includephotoluminescence and electroluminescence. Photoluminescence issub-classified to fluorescence and phosphorescence.

A photoluminescent is generally a material which absorbs energy is inthe form of light. A fluorescent material is a material which emitslight upon return to the base state from a singlet excitation, and aphosphorescent materials is a material which emits light upon return tothe base state from a triplet excitation. In fluorescent materials, orfluorophores, the electron de-excitation occurs almost spontaneously,and the emission ceases when the source of the energy exciting thefluorophore is removed. In phosphor materials, or phosphors, theexcitation state involves a change of spin state, which decays onlyslowly. In phosphorescence, light emitted by an atom or moleculepersists after the excitation source is removed.

Photoluminescent materials are used according to various embodiments ofthe present invention for altering the color of light. Since blue lighthas a short wavelength (compared, e.g., to green or red light), andsince the light emitted by a photoluminescent material has a longerwavelength than the absorbed light, blue light generated by a bluelight-emitting element such as a light-emitting diode (LED) may bereadily converted to visible light having a longer wavelength.Accordingly, in various exemplary embodiments of the invention aspecific light profile on the exit of light into zone 26 is providedusing one or more photoluminescent layers disposed on or embedded indevice 10.

The term “photoluminescent layer” is commonly used herein to describeone photoluminescent layer or a plurality of photoluminescent layers.Additionally, a photoluminescent layer may include one or more types ofphotoluminescent species. In any event, a photoluminescent layer ischaracterized by an absorption spectrum (i. e., a range of wavelengthsof light absorbed by the photoluminescent molecules to effect quantumtransition to a higher energy level) and an emission spectrum (i.e., arange of wavelengths of light emitted by the photoluminescent moleculesas a result of quantum transition to a lower energy level). The emissionspectrum of the photoluminescent layer is typically wider and shiftedrelative to its absorption spectrum. The difference in wavelengthbetween the apex of the absorption and emission spectra of thephotoluminescent layer is referred to as the Stokes shift of thephotoluminescent layer.

The absorption spectrum of the photoluminescent layer preferablyoverlaps, at least partially, the emission spectrum of the light sourcewhich feeds device 10. More preferably, for each characteristic emissionspectrum of the light source, there is at least one photoluminescentlayer having an absorption spectrum overlapping the characteristicemission spectrum. According to some embodiments of the presentinvention, the apex of the source's emission spectrum lies in thespectrum of the photoluminescent layer, and/or the apex of thephotoluminescent layer's absorption spectrum lies in the spectrum of thelight source.

The photoluminescent layer may “convert” the wavelength of a portion ofthe light emitted by the light source. More specifically, for eachphoton which is successfully absorbed by the layer, a new photon isemitted. Depending on the type of photoluminescent, the emitted photonmay have a wavelength which is longer or shorter than the wavelength ofthe absorbed photon. Photons which do not interact with thephotoluminescent layer propagate therethrough. The combination ofconverted light and non-converted light forms the profile of lightentering zone 26. This “mixed” light is preferably spectrally differentfrom each of the converted light and the non-converted light. Since themixed light is formed by the superposition of the converted light andthe non-converted light, the spectrum of the mixed light generallycontains all of the wavelengths of the converted light and thenon-converted light.

In preferred embodiments, the photoluminescent material is disposedneither on an outer surface of device 10 nor directly on alight-emitting element 34. Rather, as described further below, thephotoluminescent material (e.g., in the form of particles and/or a layeror layers) is disposed within device 10 some distance away fromlight-emitting element 34.

FIGS. 3 a-d are fragmentary schematic illustrations of device 10 showinga cross-section of zone 30 parallel to the z-x plane. Several componentsof device 10 are omitted from FIGS. 3 a-d for clarity of presentation.FIG. 3 a illustrates an embodiment in which the elongated structures atthe ends of zone 30 (structures 18-1 and 18-3, in the present example)include or consist essentially of photoluminescent material, e.g., aphosphor or a fluorophore. FIG. 3 b illustrates an embodiment in whichone or more of the inner elongated structures of zone 30 (structure18-2, in the present example) include or consist essentially ofphotoluminescent material. FIG. 3 c is a schematic illustration of anembodiment in which a photoluminescent layer 38, which may include orconsist essentially of a photoluminescent material such as a phosphor ora fluorophore, is disposed on the surface of layer 12 and/or layer 14.In this embodiment, the wavelength of the light is changed via themultiple impingements of the light on surface of layer 12 and/or 14. Inan embodiment, only one of the surfaces is coated by thephotoluminescent layer 38. For example, the surface of layer 14 may becoated by the photoluminescent layer 38 and the surface of layer 12 maybe left exposed for better light coupling between layer 12 and thelight-emitting element or funnel 32.

Photoluminescent material may also be incorporated in the form ofparticles, as illustrated in FIG. 3 d. A plurality of photoluminescentparticles 128 may be distributed within one or more of the corestructures 18 in accordance with the desired light output profile. Forexample, in one embodiment, the particles 128 are uniformly distributedin all the core structures 18. In another embodiment, the particles aredistributed such that there are core structures 18 with a higherpopulation of the particles 128 and core structures 18 with a lowerpopulation of the particles 128, depending on the desired profile in ornear each core structure.

A cross-sectional view of an exemplary embodiment of optical funnel 32is illustrated in FIG. 4 a. Optical funnel 32 receives the light fromone or more light-emitting elements 34 and distributes it prior to entryof the light into layer 12 (not shown in FIG. 4, see FIGS. 1 d and 1 e)so as to establish a plurality of entry locations within zone 30 (henceimproving the uniformity of light distribution within zone 30).Light-emitting elements 34 may be arranged near funnel 32 or they may beembedded in funnel 32. Efficient optical transmission between funnel 32and layer 12 is preferably ensured by impedance matching therebetween.Each light-emitting element 34 may be a discrete light source, e.g., anLED. In various embodiments, each light-emitting element 34 is asubstantially unpackaged (or “bare”) LED die. In such embodiments,funnel 32 or other portions of device 10 (such as zone 30, as describedfurther below) function as the “package” for light-emitting element 34.In preferred embodiments of the invention, bare LED dies do not includea phosphor or other photoluminescent material as a portion thereof(e.g., on a common substrate therewith or incorporated into or onto theLED semiconductor layer structure). Where a single light-emittingelement 34 is described herein, more than one light-emitting element 34could generally also be utilized, and vice versa. Generally, light isemitted from light-emitting element 34 upon supply of electrical currentthereto.

Funnel 32 may be made as a surface-emitting waveguide orsurface-emitting optical cavity which receives the light generated bylight-emitting elements 34 through an entry surface 142, distributes itwithin an internal volume 148, and emits it through an exit surface 144,which is typically opposite to the entry surface 142.

In some embodiments of the present invention, funnel 32 comprises one ormore light reflectors 146, which are typically arranged peripherallyabout volume 148 so as to form an optical cavity or an optical resonatorwithin volume 148. One or more light reflectors 146 may also be formedon or attached to the entry surface 142 of funnel 32. In thisembodiment, one or more openings 150 are formed on the reflectors 146 atthe entry surface, thus allowing light to enter volume 148. Openings 150may be substantially aligned, e.g., in the x-y plane, withlight-emitting elements 34.

Funnel 32 may include or consist essentially of a waveguide material, orit may be filled with a medium having a small absorption coefficient tothe spectrum or spectra emitted by the light-emitting elements 34. Forexample, funnel 32 may be filled with air, or be made of a waveguidematerial which is similar or identical to the material of the claddinglayers 12 and/or 14. The advantage of using air is its low absorptioncoefficient, and the advantage of a waveguide material identical tomaterial of the cladding layers 12, 14 is impedance matching therewith.

When funnel 32 is filled with medium having a small absorptioncoefficient (e.g., air), there may be no impedance matching at exitsurface 144 of funnel 32. Thus, some reflections and refraction eventsmay occur upon the impingement of light on the interface between funnel32 and the cladding layer 12. Neither refraction nor reflection eventscause significant optical losses; refraction events contribute to thedistribution of light within zone 30, and reflection events contributeto the distribution of light within volume 148.

In various exemplary embodiments of the invention, funnel 32 issupplemented by photoluminescent material for controlling the outputprofile of the light, as schematically illustrated in FIGS. 4 b-4 d. Forclarity of presentation, the reflectors 146 are not shown in FIGS. 4 b-4d. In any of the embodiments, funnel 32 may include one or more lightreflectors 146 as detailed above. In the embodiment illustrated in FIG.4 b, a photoluminescent layer 38 is interposed between layer 12 andfunnel 32; in the embodiment illustrated in FIG. 4 c, photoluminescentlayer 38 is embedded in funnel 32; and in the embodiment illustrated inFIG. 4 d a plurality of photoluminescent particles 128 is distributedwithin funnel 32.

Various embodiments of the present invention feature one or morelight-emitting elements 34 embedded within zone 30 of device 10 and/orphotoluminescent material (e.g., photoluminescent layer 38 and/orparticles 128) disposed within device 10 outside of the direct“line-of-sight” from light-emitting elements 34. That is, in suchembodiments, there is no direct, straight-line optical path between thelight-emitting elements 34 and the photoluminescent material; rather,light emitted from light-emitting elements 34 reflects from a reflector,a surface, or an interface within device 10 before reaching thephotoluminescent material. Thus, any light striking and beingback-reflected from the photoluminescent material will not propagatedirectly back into light-emitting element 34 (where it could beabsorbed, thus reducing overall light output and efficiency of device10). Rather, light reflecting from the photoluminescent material willtend to remain within device 10 and eventually reflected back towardzone 28 to be out-coupled. In some embodiments, there is substantiallyno direct line-of-sight between light-emitting element 34 and thephotoluminescent material, i.e., less than approximately 5% of the lightfrom light-emitting element 34 has a direct line-of-sight to thephotoluminescent material; any losses thereof are therefore negligible.

Whether or not the photoluminescent material is within a directline-of-sight of light-emitting element 34, the photoluminescentmaterial may advantageously be located remotely in relation tolight-emitting element 34, i.e., it may be present in zone 26 and/orzone 28 rather than proximate light-emitting element 34 (in zone 30 orin funnel 32, for example). The quantum efficiency (or other performancemetric) of the photoluminescent material may degrade when the materialis exposed to elevated temperatures, e.g., temperatures greater thanapproximately 50° C. Remote placement of the photoluminescent materialprevents the temperature of the material from rising during operationdue to, e.g., heat given off by light-emitting element 34. Instead, thetemperature of remotely placed luminescent material will generallyremain at the ambient temperature of the surroundings of device 10.Generally, the temperature of the luminescent material may remain atleast approximately 30° C., or even at least 100° C. less than thetemperature of light-emitting element 34 during operation.

During assembly of device 10, elevated temperatures capable of damaging(e.g., degrading the quantum efficiency of) the photoluminescentmaterial are often required when affixing or embedding light-emittingelement 34 into device 10. Remote placement of the photoluminescentmaterial enables the photoluminescent material to be provided withindevice 10 prior to the addition of light-emitting element 34—thedistance therebetween prevents the elevated temperatures from damagingthe photoluminescent material.

A remotely placed photoluminescent material may be located in any one ormore of a variety of locations, as depicted in FIGS. 4 e-4 j. FIG. 4 edepicts a photoluminescent layer 38 within zone 26 and outside thedirect line-of-sight of light-emitting element(s) in funnel 32 (e.g., asillustrated in FIG. 4 a). At least a portion of the light propagatingthrough zone 26 is converted by photoluminescent layer 38 to light of adifferent wavelength, and then the converted and unconverted lightcomponents enter zone 28 where they are out-coupled together to form,e.g., substantially white light. In this and similar configurations, thepropagating light converted by the photoluminescent material travels ina direction substantially perpendicular to the direction of the eventualout-coupled light. Such configurations may enable superior uniformity,brightness, and color of the out-coupled light.

FIG. 4 f depicts potential locations in zone 28 for the photoluminescentmaterial, which are also outside the direct line-of-sight oflight-emitting element(s) in funnel 32. First, photoluminescentparticles 128 may be utilized in conjunction with (or instead of)particles 20; at least a portion of light striking particles 128 isconverted to light of a different wavelength, and the light out-coupledfrom zone 28 is, e.g., substantially white. Additionally (or instead),photoluminescent layer 38 may be disposed within zone 28, e.g.,proximate a top edge thereof. In this configuration, at least a portionof the light already being out-coupled (i.e., on its way out of device10) is converted to light of a different wavelength. The exitingconverted and unconverted light mix to form, e.g., substantially whitelight. In configurations featuring particles 20 (or other opticalelement(s)) disposed between light-emitting element 34 and aphotoluminescent material (e.g., photoluminescent layer 38 disposedalong the top edge of zone 28), the uniformity of the light striking thephotoluminescent material may be greater than the uniformity of thelight striking particles 20. That is, the scattering by particles 20increases the uniformity of the light, which then strikes thephotoluminescent material and is out-coupled from device 10 with a highlevel of uniformity. The line of sight between light-emitting element 34and the photoluminescent material may not be eliminated by placement ofparticles 20 therebetween, as some light may propagate through theregion populated with particles 20 without being scattered thereby.

FIG. 4 g depicts possible locations for a photoluminescent materialdescribed with reference to FIGS. 4 e and 4 f, any of which (or anycombination of which) may be utilized in conjunction with a device 10shaped to eliminate the direct line-of-sight between the light-emittingelement(s) in funnel 32 and photoluminescent layer 38 and/or particles128. As shown in FIG. 4 g, device 10 may include a bend, curve, or othergeometry in zone 26 (or even in zone 28) which facilitates theelimination of a direct line-of-sight between the light-emittingelement(s) and the photoluminescent material. This geometry may alsofacilitate subsequent “tiling” of multiple devices 10 to form anillumination panel, e.g., a panel in which the zones 28 of devices 10overlie zones 26 and/or 30 of adjacent devices 10 (as further describedbelow with reference to FIGS. 12 a and 12 b). The shape depicted in FIG.4 g is exemplary, and many other configurations are possible.

FIGS. 4 h-4 j are analogous to FIGS. 4 e-4 g, respectively, but depictone or more light-emitting elements 34 embedded within device 10 (hereshown embedded within a core structure 18 of zone 30) rather thancoupled to device 10 via funnel 32. As shown by the schematic breakwithin zone 26 in FIGS. 4 h-4 j, zone 26 may be elongated and/or besized and shaped so as to substantially or completely eliminate thedirect line-of-sight between light-emitting element(s) 34 andphotoluminescent layer 38 and/or particles 128. Each device 10 depictedin FIGS. 4 e-4 j may also incorporate cladding layers 12,14, e.g., asillustrated in FIGS. 1 c-1 e.

In a preferred embodiment, light from light-emitting element 34 (whetherembedded within device 10 or operated in conjunction with funnel 32)generally enters zone 30 in an “in-coupling direction,” i.e., along thez axis indicated in FIG. 1 b. Once in-coupled into device 10 byscattering from particles 20 and/or reflector 36, the light generallypropagates through device 10 (e.g., through zone 26) in a “propagationdirection” that is substantially perpendicular to the in-couplingdirection. As illustrated in FIG. 1 b, the propagation direction isgenerally along the x axis. After the light enters zone 28, it isgenerally out-coupled from device 10 (i.e., emitted from surface 23and/or 24) in an “out-coupling direction” that is substantiallyperpendicular to the propagation direction (e.g., along the z axisindicated in FIG. 1 b). Thus, the in-coupling direction and theout-coupling direction may be substantially parallel. In someembodiments in which photoluminescent layer 38 and/or particles 128 arepresent, at least a portion of the light propagating in device 10 in thepropagation direction is stimulated by photoluminescent layer 38 and/orparticles 128, giving rise to the mixed light that is out-coupled fromdevice 10 in an out-coupling direction substantially perpendicular tothe propagation direction. This configuration may enable betterbrightness and/or color uniformity than devices in which stimulatedlight (i.e., light before or as it strikes a photoluminescent material)propagates in a direction that is not substantially perpendicular (e.g.,a substantially parallel direction) to an out-coupling direction of themixed light resulting from stimulation by the photoluminescent material.

Phosphors are widely used for coating individual LEDs, typically toobtain white light therefrom. However, photoluminescent layersincorporated in waveguide devices as described herein have not beenemployed. The advantage of providing photoluminescent layer 38 and/orparticles 128 (in layer 16 and/or funnel 32) as opposed to on eachindividual light-emitting element, is that waveguide device 10 diffusesthe light before emitting it. Thus, instead of collecting light from apoint light source (e.g., a LED), photoluminescent layer 38 and/orparticles 128 collects light having a predetermined extent. Thisconfiguration allows a better control on the light profile provided bydevice 10.

Many types of phosphorescent and fluorescent substance are contemplated.Representative examples include, without limitation, the phosphorsdisclosed in U.S. Pat. Nos. 5,813,752, 5,813,753, 5,847,507, 5,959,316,6,155,699, 6,351,069, 6,501,100, 6,501,102, 6,522,065, 6,614,179,6,621,211, 6,635,363, 6,635,987, 6,680,004, 6,765,237, 6,853,131,6,890,234, 6,917,057, 6,939,481, 6,982,522, 7,015,510, 7,026,756,7,045,826, and 7,005,086, the entire disclosures of which are herebyincorporated by reference. In an embodiment, the quantum efficiency ofphotoluminescent layer 38 and/or particles 128 is only stable up to atemperature of approximately 50° C. However, in many configurations thetemperature of such materials remains lower than this level due tospatial separation of photoluminescent layer 38 and/or particles 128from the light-emitting element(s). In various embodiments, layer 38and/or particles 128 include or consist essentially of one or moreelectroluminescent materials rather than (or in addition to)photoluminescent materials. Such electroluminescent materials mayinclude or consist essentially of quantum dot materials and/or organicLED (OLED) materials. Suitable quantum dots may include or consistessentially of cadmium selenide.

There is more than one configuration in which photoluminescent layer 38may be used. In one embodiment, photoluminescent layer 38 complementsthe light emitted by light-emitting elements 34 to create a white light,e.g., using dichromatic, trichromatic, tetrachromatic or multichromaticapproach. For example, a blue-yellow dichromatic approach may beemployed, in which case blue light-emitting elements (e.g., InGaN LEDswith a peak emission wavelength at about 460 nm) are used, andphotoluminescent layer 38 may include or consist essentially of phosphormolecules with an absorption spectrum in the blue range and an emissionspectrum extending to the yellow range (e.g., cerium-activated yttriumaluminum garnet, or strontium silicate europium). Since the scatteringangle of light sharply depends on the frequency of the light(fourth-power dependence for Rayleigh scattering, or second-powerdependence for Mie scattering), the blue light generated by the bluelight-emitting elements 34 is efficiently diffused in the waveguidematerial before interacting with photoluminescent layer 38 and/orparticles 128. Layer 38 and/or particles 128 emit light in its emissionspectrum and complement the blue light which is not absorbed byphotoluminescent layer 38 and/or particles 128 to white light.

In another dichromatic configuration, ultraviolet light-emittingelements (e.g., LEDs of GaN, AlGaN, and/or InGaN with a peak emissionwavelength between 360 nm and 420 nm) are used. Light of suchultraviolet light-emitting elements is efficiently diffused in thewaveguide material. To provide substantially white light, twophotoluminescent layers 38 and/or two types of photoluminescentparticles 128 are preferably employed. One such photoluminescent layerand/or type of particles may be characterized by an absorption spectrumin the ultraviolet range and emission spectrum in the orange range (withpeak emission wavelength from about 570 nm to about 620 nm), and anotherphotoluminescent layer and/or type of particles may be characterized byan absorption spectrum in the ultraviolet range and emission spectrum inthe blue-green range (with peak emission wavelength from about 480 nm toabout 500 nm). The orange light and blue-green light emitted by the twophotoluminescent layers 38 and/or two types of photoluminescentparticles 128 blend to appear as white light to an observer. Since thelight emitted by the ultraviolet light-emitting elements is above orclose to the end of the visual range, it is not discerned by theobserver. When two photoluminescent layers 38 are employed, they may bedeposited one on top of the other so as to improve the uniformity.Alternatively, a single photoluminescent layer 38 having two types ofphotoluminescent material with the above emission spectra may beutilized.

In another embodiment a trichromatic approach is employed. For example,blue light-emitting elements may be employed as described above, withtwo photoluminescent layers 38 and/or two types of photoluminescentparticles 128. A first photoluminescent layer 38 and/or type ofphotoluminescent particles 128 may include or consist essentially ofphosphor molecules with an absorption spectrum in the blue range and anemission spectrum extending to the yellow range as described above, anda second photoluminescent layer 38 and/or type of photoluminescentparticles 128 may include or consist essentially of phosphor moleculeswith an absorption spectrum in the blue range and an emission spectrumextending to the red range (e.g., cerium-activated yttrium aluminumgarnet doped with a trivalent ion of praseodymium, or europium-activatedstrontium sulphide). The unabsorbed blue light, the yellow light, andthe red light blend to appear as white light to an observer.

Also contemplated is a configuration is which light-emitting elements 34with different emission spectra are employed and severalphotoluminescent layers 38 are deposited and/or several types ofphotoluminescent particles 128 are distributed, such that the absorptionspectrum of each photoluminescent layer 38 and/or type ofphotoluminescent particles 128 overlaps one of the emission spectra ofthe light-emitting elements 34, and all the emitted colors (of thelight-emitting elements 34 and the photoluminescent layers 38 and/orparticles 128) blend to appear as white light. The advantage of such amulti-chromatic configuration is that it provides a high-quality whitebalance because it allows better control of the various spectralcomponents of the light in a localized manner, e.g., along an edge orsurface of device 10.

The color composite of the white output light depends on the intensitiesand spectral distributions of the emanating light emissions. Thesedepend on the spectral characteristics and spatial distribution of thelight-emitting elements 34, and, in the embodiments in which one or morephotoluminescent components (layers 38 and/or particles 128) areemployed, on the spectral characteristics of the photoluminescentcomponents and on the amount of unabsorbed light. The amount of lightunabsorbed by the photoluminescent components is, in turn, a function ofthe characteristics of the components, e.g., thickness of thephotoluminescent layer(s) 38, density of photoluminescent material(s),and the like. By judiciously selecting the emission spectra oflight-emitting element 34 and optionally the thickness, density, andspectral characteristics (absorption and emission spectra) ofphotoluminescent layer 38 and/or particles 128, device 10 may providesubstantially uniform white light.

In any of the above embodiments, the “whiteness” of the light may betailored according to the specific application for which device 10 isintended. For example, when device 10 is incorporated as backlight of anLCD device, the spectral components of the light provided by device 10may be selected in accordance with the spectral characteristics of thecolor filters of the liquid crystal panel. In other words, since atypical liquid crystal panel includes an arrangement of color filtersoperating at a plurality of distinct colors, the white light provided bydevice 10 includes at least at the distinct colors of such filters. Thisconfiguration significantly improves the optical efficiency as well asthe image quality provided by the LCD device, because the optical lossesdue to mismatch between the spectral components of the backlight unitand the color filters of the liquid crystal panel are reduced oreliminated.

Thus, in the embodiment in which the white light is achieved bylight-emitting elements 34 emitting different colors of light (e.g., redlight, green light and blue light), the emission spectra of thelight-emitting elements 34 are preferably selected to substantiallyoverlap the characteristic spectra of the color filters of an LCD panel.In the embodiment in which device 10 is supplemented by one or morephotoluminescent components (layers 38 and/or particles 128), theemission spectra of the photoluminescent components and optionally theemission spectrum (or spectra) of the light-emitting elements arepreferably selected to overlap the characteristic spectra of the colorfilters of an LCD panel. Typically, the overlap between a characteristicemission spectrum and a characteristic filter spectrum is about 70%spectral overlap, more preferably about 80% spectral overlap, and evenmore preferably about 90%.

The following is a description of a production process for the corelayer 16 and the optical waveguide device 10 according to variousexemplary embodiments of the present invention.

In some embodiments, the core layer is formed by coextrusion. As usedherein, the term “coextrusion” refers to the process of simultaneousextrusion of several die outputs which are welded together beforechilling to form an extrudate having an open shape, e.g., a non-tubularsheet. An extrudate formed by a coextrusion process according to someembodiments of the present invention may be a single-layer structure ora laminate structure having two or more layers. In some embodiments ofthe present invention the coextrusion process is employed in anextrusion coating process in which an extrudate formed by thecoextrusion process is applied so as to coat one or more existinglayers.

Thus, a plurality of light-transmissive compositions in a molten orplastic state may be coextruded to form the elongated core structures ofcore layer 16. Each light-transmissive composition may be extruded toform a single core structure 18, and may be a polymeric material mixedwith light-scattering particles of type, size and concentration selectedto provide the core structure 18 with the desired optical properties(e.g., mean free path).

A coextrusion apparatus 50 which may be used according to someembodiments of the present invention is schematically illustrated inFIG. 5. As shown therein, several melt or plasticized streams 52 (threein the illustration) are individually extruded from a plurality ofextruders 54. The melt streams comprise light-transmissive compositionsin accordance with the respective core structures 18 to be formed.Extruders 54 discharge the compositions, which are conveyed byconventional conduits (not shown) to a coextrusion die or feedblock 56.Die 56 combines and arranges the compositions and issues a compositeflat stream 58 in which the various compositions flow side-by-side. Achill roller system 60 quenches stream 58 to form core layer 16 whichincludes or consists essentially of a plurality of core structures 18 asdescribed above. The formed core structures 18 may have any shape orcross-section, e.g., rectangular or triangular.

One or more of the extruded core structures 18 (e.g., the sidemostelongated structure 18-1 and 18-n, see FIG. 1 a) may be made reflective.This may be achieved by judicious selection of the composition fromwhich these core structures are formed. For example, a compositioncharacterized by high refractive index (e.g., 2 or more) may be fed tothe respective extruder 54. A representative example of a materialhaving a sufficiently high refractive index for reflectivity is TiO₂,which has a refractive index of about 2.5. Also contemplated, is the useor incorporation of a substantially opaque composition or theincorporation of reflective particles at a sufficiently high density tomake core structure 18 reflective.

Coextrusion apparatus 50 may be adapted to simultaneously form the corelayer 16 as well as the cladding layers 12, 14. This approach isillustrated in FIG. 6, which shows apparatus 50 with a die 56 configuredto combine and arrange the compositions into a laminated flat stream 58in which the intermediate layer of stream 58 is composed of side-by-sideflow of the various compositions of the core layer 16 and the outerlayers of the stream are composed of the compositions of the claddinglayers 12, 14. In an embodiment, additional layers are formed above andbelow the cladding layers 12, 14, e.g., for the purpose of protecting orreenforcing the cladding layers 12, 14.

An alternative embodiment is illustrated in FIGS. 7 a and 7 b. In thisembodiment, apparatus 50 performs an extrusion coating process, wherebythe core structures 18 of core layer 16 are coextruded on a claddinglayer 12 which is already in a dimensionally stable (i.e., rigid) state.Once core layer 16 is dimensionally stable (e.g., following cooling, ortreatment with roller system 60), cladding layer 14 may optionally belaminated on core layer 16 to form a three-layer structure.

Once the core layer 16 and optionally the cladding layers 12, 14 arecoextruded, the layer(s) may be further treated while the compositionsare in molten or plastic state. One example of such treatment isapplication of heat and/or pressure so as to at least partially mixrespective compositions at common edges of adjacent core structures 18.When the adjacent core structures 18 have different particleconcentrations (including the case of two adjacent structures in whichone has a zero concentration and the other has a non-zeroconcentration), the heat and/or pressure treatment may result in aconcentration gradient across the lateral direction of the corestructures 18. This embodiment is particularly useful when it is desiredto have a smooth profile along the optical mean free path.Post-extrusion treatment of the formed core structures 18 may beperformed by roller system 60 (prior to the cooling of the extrudedstructures), or it may be done using another roller system. When thecompositions comprise thermoplastic materials, the post- extrusiontreatment may be performed after the structures are cooled. In thisembodiment, the post-extrusion treatment may include reheating of thecore structures.

The optical waveguide device featured in embodiments of the presentinvention may also be manufactured by a lamination process. Suitablelamination processes may be employed on both thermoset andtheremoplastic materials. Any lamination technique suitable for thematerials from which core layer 16 and cladding layers 12, 14 are formedmay be employed. The lamination process may be executed with or withouta solid support. When a solid support (e.g., a metal support or otherrigid support) is employed, it is preferably designed and constructed toallow lamination of individual core structures 18 in a side-by-sidefashion. Thus, the solid support preferably fixes each individual corestructure 18 to its place sidewise with a previously laminated elongatedstructure.

A lamination technique according to various embodiments is schematicallyillustrated in FIGS. 8 a-8 c. The process starts with a substrate 62(FIG. 8 a), on which the lamination process is executed. The processcontinues with the lamination of a plurality of core structures 18(e.g., elongated core structures) in a side-by-side configuration on asubstrate 62 to form core layer 16 (FIG. 8 b). The lamination may beperformed by heat-and-press, with or without adhesives. Optionally, butnot obligatorily, substrate 62 may serve as a cladding layer (e.g.,layer 12 of FIGS. 1 c-1 e or 1 g). In this embodiment, substrate 62preferably includes or consists essentially of a flexible claddingmaterial and is preferably laid on a support substrate (not shown),which is desirably planar.

One or more light-reflective structures may be laminated sidewiserelative to core layer 16. This may be done in a similar manner to thelamination of the other core structures 18.

Once laminated side-by-side, the core structures 18 may be joined attheir common ends using any technique known in the art, including,without limitation, adhesive bonding, solvent bonding, or welding (alsoknown as fusion bonding). The lamination of core 16 on substrate 62 maybe preceded by a step in which an adhesive optical material is appliedon substrate 62. If desired, substrate 62 may be removed following thelamination of the core structures 18. It this embodiment, the air servesas the “cladding” layer as detailed above.

In various exemplary embodiments of the invention the process continuesby laminating cladding layer 14 on core layer 16 (FIG. 8 c). Optionally,an optical adhesive may be applied on core layer 16 prior to thelamination of cladding layer 14 thereon.

An additional technique for fabricating device 10 is illustrated inFIGS. 9 a-9 c. The process starts with substrate 62 (FIG. 9 a). Aplurality of core structures 18 having the shape of plaques are tiled ina side-by-side configuration on a substrate 62 to form core layer 16(FIG. 9 b). The tiling may be performed by lamination techniques such asheat-and-press, with or without adhesives. Optionally, but notobligatorily, substrate 62 may serve as a cladding layer (e.g., layer 12of FIGS. 1 c-e or 1 g). In this embodiment, substrate 62 is made of aflexible cladding material and is preferably laid on a support substrate(not shown), which is preferably planar.

One or more light-reflective structures may be laminated sidewiserelative to core layer 16. This may be done in a similar manner to thelamination of the other core structures 18.

Once laminated side-by-side, the core structures 18 may be joined attheir common ends using any technique known in the art, including,without limitation, adhesive bonding, solvent bonding, or welding. Thelamination of core 16 on substrate 62 may be preceded by a step in whichan adhesive optical material is applied on substrate 62. If desired,substrate 62 may be removed following the lamination of the corestructures 18. In this embodiment, the air serves as the “cladding”layer as detailed above. In various exemplary embodiments of theinvention the process continues by laminating cladding layer 14 on corelayer 16 (FIG. 9 c). Optionally, an optical adhesive may be applied oncore layer 16 prior to the lamination of cladding layer 14 thereon.

Following the lamination process of any of the above embodiments, one ormore additional layers (not shown) may be attached to cladding layers 12and/or 14. This may be achieved using any procedure known in the art,including, without limitation, printing, embossing, lamination, and thelike. The attachment of the additional layers may be performed using anytechnique, including, without limitation, adhesive bonding, solventbonding, welding, mechanical fastening, co-consolidation, and the like.The additional layer may cover the entire surface area of the claddingor a portion thereof. For example, a reflective foil 36 (see, e.g., FIG.1 a) may be attached to cladding layer 14. Also contemplated are jacketlayers for protecting the cladding layers 12, 14.

An additional technique for fabricating device 10 according to someembodiments of the present invention is illustrated in FIGS. 10 a-10 c.In these embodiments co-injection molding is employed. Co-injectionmolding is a variant of a process known as injection molding. Ininjection molding thermoplastic polymers or the like are fed from ahopper into a barrel, melted by a reciprocating screw and/or electricheat, and are propelled forward by a ram (piston, plunger) or the screw(used as a plunger) into a mold cavity, which is cooled to below theheat-distortion temperature of the resin.

Co-injection molding takes advantage of a characteristic of injectionmolding called fountain flow. As the cavity is filled, the material atthe melt front moves from the center line of the stream to the cavitywalls. The walls are typically kept below the transition temperature ofthe melt such that the material that touches the walls cools rapidly andfreezes in place. This provides insulating layers through which new meltmakes its way to the melt front.

In some embodiments of the present invention, the co-injection techniqueis employed for forming a core layer 16 having a plurality of corestructures 18 in a nested configuration. A co-injection molding systemsuitable for the present embodiments is illustrated in FIG. 10 a. Thesystem typically includes a co-injection manifold 230 mounted relativeto a mold cavity 220, and shaped according to the desired shape of thedevice. In various exemplary embodiments of the invention, mold cavity220 has a substantially planar shape.

Manifold 230 includes a nozzle housing 234 having forward and rearwardends. The illustrated nozzle housing 234 is generally V-shaped, but anyother shape suitable for co-injection may be utilized. Nozzle housing234 includes a plurality of arms 254, each having a rearward end 262,and includes an outwardly extending mounting portion 266. Arms 254 aresupported by mounting columns 236, which are typically fixedly mountedon a horizontal surface of a machine base sled (not shown).

Housing 234 has an outlet 270 in its forward end, as well as a pluralityof inlets 274 in the rearward end of each arm. Outlet 270 communicateswith an inlet 226 of cavity 220. Inlets 274 of housing 234 respectivelycommunicate with a plurality of injection nozzles 284 of respectiveinjection units (not shown). Each injection nozzle is typically fed by adifferent light-transmissive composition as described above.

Manifold 230 also includes a valve 258 movable between a plurality ofpositions. In each position, valve 258 open a fluid communicationchannel between one of inlets 274 and outlet 270. Also contemplated is aposition in which valve 258 closes all communication channels. Valve 258may be moved relative to housing 234 by a hydraulic cylinder 278 mountedon the manifold 230.

The co-injection system may operate as follows. The nozzle housing isoriented such that each injection nozzle provides one type oflight-transmissive composition. The co-injection process begins with thevalve 258 in a position selected such that a first light-transmissivecomposition (e.g., a composition with low concentration oflight-scattering particles), in a molten or plastic state, flows throughthe outlet 270. The selected composition is injected into the moldcavity 220. The valve 258 is then moved to another position to allowflow of a second light-transmissive composition (e.g., a compositionwith a higher concentration of light-scattering particles), in a moltenor plastic state, through the outlet 270. By the effect of fountain flowdescribed above, the second composition is nested into the firstcomposition. The process is optionally continued by repositioning thevalve 258 so as to inject into the mold a third composition in a moltenor plastic state. The third composition is nested into the previouslyinjected second composition. The third composition may have aconcentration of light-scattering particles higher than that of thesecond composition.

Any number of light-transmissive compositions may be serially injectedinto the mold so as to form a core layer 16 with a plurality of corestructures 18 (which may be flexible) joined in a nested configuration.The melt fronts of the different light-transmissive compositions aredesignated in FIG. 10 a by reference numerals 222-1, . . . , 222-n. Thepropagation of each melt front nesting into previously injectedlight-transmissive compositions is shown by arrows. An advantage ofusing a co-injection manifold for manufacturing the core layer 16 isthat it allows more flexibility in selecting the characteristics of thedifferent core structures 18. A continuous or semi-continuous control onthe operation of the co-injection manifold may facilitate formation ofcore structures 18 in a manner such that the characteristic mean freepath varies substantially smoothly from one core structure 18 to theother. Since the effective refractive index varies with thecharacteristic mean free path, various embodiments of the presentinvention allow production of an optical waveguide device 10 having agraded effective refractive index along the lateral direction.

Once the core layer 16 is formed, it is typically released from themold. A top view of the core layer 16 once released from the mold isillustrated in FIG. 10 b. The procedure optionally and preferablycontinues by cutting the core layer 16 along the lateral direction so asto remove one or more marginal regions 228 therefrom, thereby providinga core layer 16 in which the core structures 18 are joined in aside-by-side configuration. Shown in FIG. 10 b are two cut lines 224parallel to the lateral direction along which the core layer may be cut.A top view of the core layer 16 once cut along cut lines 224 isillustrated in FIG. 10 c. The procedure may continue to form additionallayers such as cladding layers 12, 14, and/or photoluminescent layers 38on the core structure 16 as detailed above. In some embodiments of thepresent invention, the co-injection system is configured to inject alsothe cladding layers 12, 14.

Referring to FIG. 11, in various embodiments of the invention,multilayer optical waveguide device 100 comprises multiple waveguidedevices 10, each of which may be fabricated as described above. Thedevices 10 in multilayer device 100 may be disposed in a vertically“stacked” configuration as depicted in FIG. 11. A layer 1100 oflow-refractive-index material may be disposed between each “layer” 10 inorder to prevent undesired light propagation from one layer 10 toanother. Light may be coupled in to zone 30 of each layer 10 from adifferent light source, or the same light source may be utilized foreach layer 10. In a particular embodiment, multilayer device 100provides controllable RGB illumination by including different types ofphotoluminescent particles 128 in each layer 10. For example, a bottomlayer 10 may include photoluminescent particles 128 that emit red light,a middle layer 10 may include photoluminescent particles 128 that emitgreen light, and a top layer 10 may include photoluminescent particles128 that emit blue light. Such a multicolor multilayer device 100 may besuitable for LCD backlight applications. As shown in FIG. 11, the zones28 of the layers 10 may be substantially vertically aligned such thatlight emitted from the bottom layer 10 travels through the other layers10 before finally being emitted from multilayer device 100. In otherwords, each zone 28 in multilayer device 100 may have substantially novertical overlap with zones 26, 30 of the other layers 10.

Referring to FIGS. 12 a and 12 b, in various embodiments of theinvention, multiple optical waveguide devices 10 are utilized togetherto provide enhanced functionality. Illumination panel 1200 includes orconsists essentially of a plurality of optical waveguide devices 10attached together at their edges (or overlapped) in a “tiled” fashion.In order to provide substantially uniform illumination across the entiresurface of illumination panel 1200, the waveguide devices 10 may betiled together such that only out-coupling region 28 of each device 10is visible. In-coupling region 30 and propagation region 26 of eachdevice 10 may therefore be disposed beneath adjoining devices 10 and notvisible. While illumination panel 1200 is illustrated as substantiallyplanar, the flexibility of each waveguide device 10 enables illuminationpanel 1200 to be configured in a variety of shapes, including curvedsheets and even spheres.

Illumination panel 1200 may be utilized to provide substantially uniformillumination in a variety of applications. For example, illuminationpanel 1200 may itself be utilized as a luminaire for lightingapplications. In another embodiment, illumination panel 1200 is utilizedas a backlight unit for a display device 1210, e.g., a liquid crystaldisplay (LCD). Display device 1210 may additionally include an LCD panel1220 defining a plurality of pixels, and may be actuated by signalsreceived from control circuitry 1230.

Referring to FIG. 13, a phosphor layer 1302 may be added to alight-emitting element 1300. The phosphor layer 1302 converts lightemitted from the in-coupling region 1304 from the light source 1306,such as an LED, into a different color (i.e., changes the spectrum). Forexample, part of the light from a blue LED may be converted to yellowlight, which mixes with the remaining blue light to provide white outputillumination. In other embodiments, phosphor material is placed at anylocation in the optical path, including locations without any directline of sight from any light source.

The waveguide materials from which the waveguide device 10 is made mayinclude or consist essentially of one or more polymeric materials. Thepolymeric material may optionally include a rubbery or rubber-likematerial. The material may be formed by dip-molding in a dipping medium,for example, a hydrocarbon solvent in which a rubbery material isdissolved or dispersed. The polymeric material optionally and preferablyhas a predetermined level of cross-linking, which is preferably betweenparticular limits. The cross-linking may optionally be physicalcross-linking, chemical cross-linking, or a combination thereof. Anon-limiting illustrative example of a chemically cross-linked polymeris cross-linked polyisoprene rubber. Non-limiting illustrative examplesof physically cross-linked polymers include cross-linked blockco-polymers and segmented co-polymers, which may be cross-linked due to,e.g., micro-phase separation. The material is optionally cross-linkedthrough application of radiation, such as, but not limited to, electronbeam radiation and/or electromagnetic (e.g., ultraviolet) radiation.

Although not limited to rubber itself, the material optionally andpreferably has the physical characteristics (e.g., parameters relatingto tensile strength and elasticity) of rubber. For example, thewaveguide material may be characterized by a tensile set value which isbelow 5%. The tensile set value generally depends on the degree ofcross-linking and is a measure of the ability of a flexible material,after having been stretched either by inflation or by an externallyapplied force, to return to its original dimensions upon deflation orremoval of the applied force.

The tensile set value may be determined by, for example, placing tworeference marks on a strip of the waveguide material and noting thedistance between them, stretching the strip to a certain degree, forexample, by increasing its elongation to 90% of its expected ultimateelongation, holding the stretch for a certain period of time, e.g., oneminute, then releasing the strip and allowing it to return to itsrelaxed length, and re-measuring the distance between the two referencemarks. The tensile set value is then determined by comparing themeasurements before and after the stretch, subtracting one from theother, and dividing the difference by the measurement taken before thestretch. In a preferred embodiment, using a stretch of 90% of anexpected ultimate elongation and a holding time of one minute, thepreferred tensile set value is less than 5%. Also contemplated arematerials having about 30% plastic elongation and less than 5% elasticelongation.

Other exemplary materials, which may optionally be used alone or incombination with each other, or with one or more of the above rubbermaterials, include but are not limited to, crosslinked polymers such as:polyolefins, including but not limited to, polyisoprene, polybutadiene,ethylene-propylene copolymers, chlorinated olefins such aspolychloroprene (neoprene) block copolymers, including diblock-,triblock-, multiblock- or star-block-, such as:styrene-butadiene-styrene copolymers, or styrene-isoprene-styrenecopolymers (preferably with styrene content from about 1% to about 37%),segmented copolymers such as polyurethanes, polyether-urethanes,segmented polyether copolymers, silicone polymers, including copolymers,and fluorinated polymers and copolymers. In some embodiments of thepresent invention, the waveguide material may include or consistessentially of IOTEK.

The embedded particles may be glass beads, BaSO₄ particles, and/orsimilar particles. The volume density of the particles may be from about0.1% to about 5%.

The number of extruders used to fabricate the core layer 16 may numberfrom three to approximately 10. When the cladding layers 12, 14 areformed simultaneously with the core layer 16 the number of extruders maynumber from three to approximately 15. The total width of thecoextrusion die may be about 400 mm to about 1200 mm, and it may beconstructed and designed to provide from about 20 to about 100side-by-side core structures 18.

The thickness of the cladding layers 12, 14 may be about 10 μm to about100 μm. The thickness of the core layer 16 may be about 400 μm to about1300 μm. The number of core structures 18 in the core layer may beapproximately 20 structures to approximately 100 structures. The widthof a single core structure 18 may be about 5 mm to about 30 mm.

EXAMPLES

The core structures 18 of an optical waveguide device 10 were fabricatedfrom polyurethane. Two outer in-coupling zones 30 were eachapproximately 22 mm wide and included 0.5% volume density of VELVOLUX Msynthetic BaSO₄ particles (available from Sachtleben Chemie GmbH ofDuisburg, Germany) having approximate diameters of 5 μm. Propagationzones 26 were each approximately 29 mm wide and were substantiallyparticle-free. The center out-coupling zone 28 was approximately 77 mmwide, and was composed of three core structures 18. The outer corestructures 18 were approximately 26 mm wide and included 0.35% volumedensity of VELVOLUX M synthetic BaSO₄ particles. The middle corestructure 18 of out-coupling zone 28 was approximately 25 mm wide andcontained 0.2% volume density of BLANC FIXE F synthetic BaSO₄ particles(also available from Sachtleben Chemie GmbH of Duisburg, Germany) havingapproximate diameters of 1 μm. Out-coupling region 28 exhibited anine-point average brightness of approximately 9078 Nits, with auniformity of approximately 10%.

Another optical waveguide device 10 was fabricated from IOTEK, andincluded a propagation zone 26 that was substantially particle-free andhad a width of approximately 11 mm. Out-coupling zone 28 was composed ofthree core structures 18. In increasing distance from propagation zone26, these core structures 18 were 1) a 17 mm-wide region having 0.75%volume density of 5 μm-diameter BaSO₄ particles, 2) a 10 mm-wide regionhaving 1.5% volume density of 5 μm-diameter BaSO₄ particles, and 3) a 10mm-wider region having 3% volume density of 5 μm-diameter BaSO₄particles. Illumination from this out-coupling zone 28 was approximatelyuniform across its width.

The terms and expressions employed herein are used as terms andexpressions of description and not of limitation, and there is nointention, in the use of such terms and expressions, of excluding anyequivalents of the features shown and described or portions thereof. Inaddition, having described certain embodiments of the invention, it willbe apparent to those of ordinary skill in the art that other embodimentsincorporating the concepts disclosed herein may be used withoutdeparting from the spirit and scope of the invention. Accordingly, thedescribed embodiments are to be considered in all respects as onlyillustrative and not restrictive.

1. An illumination structure comprising: a substantially non-fiberwaveguide; and a discrete light source disposed proximate a bottomsurface of a first portion of the waveguide, wherein light is absorbedinto the structure through the bottom surface of the first portion andis emitted from a top surface of a second portion of the waveguidehaving substantially no overlap with the first portion of the waveguide.2. The illumination structure of claim 1, wherein the first and secondportions of the waveguide are spaced apart from each other, and light isemitted only from the second portion of the waveguide.
 3. Anillumination structure comprising: a substantially non-fiber waveguide;and a discrete light source disposed proximate a bottom surface of afirst portion of the waveguide, wherein a propagation direction, withina second portion of the waveguide, of light from the discrete lightsource is substantially perpendicular to an in-coupling direction of thelight.
 4. The illumination structure of claim 3, wherein the propagationdirection of the light is substantially perpendicular to an out-couplingdirection of the light in a third portion of the waveguide, theout-coupling direction being parallel to the in-coupling direction. 5.The illumination structure of claim 3, further comprising a phosphormaterial for converting some of the light to a different wavelength, theconverted light mixing with unconverted light to form mixed lightspectrally different from both the unconverted light and the convertedlight.
 6. The illumination structure of claim 5, wherein an out-couplingdirection of the mixed light is substantially perpendicular to thepropagation direction.
 7. A method of forming a substantially non-fiberwaveguide, the method comprising forming a plurality of joined corestructures, at least one of which is substantially free of scatteringparticles.
 8. The method of claim 7, wherein at least some of the corestructures comprise a plurality of scattering particles, and wherein atleast one of a size, a concentration, or a type of the scatteringparticles varies among at least two of the core structures.
 9. Themethod of claim 7, wherein forming the plurality of joined corestructures comprises at least one of co-injection molding, coextrusion,coating, lamination, bonding, or welding.
 10. The method of claim 7,further comprising forming a cladding layer on at least one of a topsurface or a bottom surface of the plurality of joined core structures.11. The method of claim 7, wherein, across a thickness thereof, there issubstantially no overlap between adjoining core structures.
 12. Themethod of claim 7, wherein the waveguide is substantially planar.